This invention relates to a drive unit for a brushless motor.
A brushless motor generally includes a stator provided thereon with a plurality of excitation windings and a rotor including a plurality of rotor magnetic poles each constituted by a permanent magnet. The brushless motor thus generally constructed is driven and controlled by a drive unit. A drive unit which has been conventionally used for this purpose generally includes a position detector for detecting a position of the rotor, a rotational speed detecting means for detecting a rotational speed of the rotor, a plurality of excitation changing-over semiconductor switches each connected in series to each of the excitation windings so as to permit an excitation current to flow therethrough to the excitation winding when it is turned on, and a drive circuit for outputting an on/off change-over signal for each of the excitation changing-over semiconductor switches depending on an output of the position detector. The brushless motor has an inherent problem that a dead point must be avoided during starting thereof.
One of approaches to the dead point problem which have been conventionally employed in the art is that imbalance of excitation torque with respect to positive torque in a rotational direction which occurs due to nonuniformity of a gap between the magnetic pole of the stator and the permanent magnet of the rotor is utilized. Another approach is that a control section of the drive circuit is provided with a function of interrupting feeding to the excitation windings in order to prevent burning of the motor during constraint thereof at the dead point.
A timing at which changing-over of phases is carried out is set by detecting a magnetic flux of the permanent magnets of the rotor by means of a hall device. However, an increase in rotational speed of the rotor causes a voltage induced across each of the excitation windings to be increased in proportion thereto. Thus, in order to permit an input current to have a readily controllable waveform, it is often carried out to vary a position of the hall device to shift the timing. This causes a region in which negative torque is generated in a rotational direction of the rotor to appear with respect to a stable point of the rotor during starting of the motor. When it is started at the point as well, a cycle which starts at reverse rotation and returns through arrival at a phase change-over point, normal rotation, arrival at the phase change-over point, reverse rotation and a phase change-over point to normal rotation is repeated. In this instance, positive torque is substantially balanced as compared with negative torque, therefore, normal rotation is normally carried out. However, the control circuit which functions to prevent burning of the motor often judges that such operation indicates abnormality in rotation of the rotor. This causes feeding to the excitation windings to be interrupted, leading to a failure in normal starting of the motor. Also, when a plurality of fan motors each constituting the brushless motor are arranged, reverse rotation of a rotor of one or more of rotors of the fan motors is often caused due to an air pressure generated by the remaining fan motors. This fails in normal rotation of the rotor when a power supply is turned on, leading to operation of the control circuit for preventing burning of the motor. This causes interruption of feeding to the excitation windings, resulting in a failure in normal starting of the motor.
The present invention has been made in view of the foregoing disadvantage of the prior art.
Accordingly, it is an object of the present invention to provide a drive unit for a brushless motor which is capable of rotating a rotor in a normal or positive direction during starting of the brushless motor.
In accordance with the present invention, a drive unit for a brushless motor which includes a stator provided thereon with a plurality of excitation windings and a rotor including a plurality of rotor magnetic poles each constituted by a permanent magnet is provided. The drive unit generally includes a position detector for detecting a position of the rotor of the brushless motor, a rotational speed detecting means for detecting a rotational speed of the rotor, a plurality of excitation change-over semiconductor switches each connected in series to each of the excitation windings so as to permit an excitation current to flow therethrough to each of the excitation windings when it is turned on, and a drive circuit for outputting an on/off change-over signal for the excitation change-over semiconductor switches depending on an output of the position detector.
One of features of the drive unit generally constructed as described above is that a power feed semiconductor switch is arranged between the excitation windings and a power supply so as to permit power to be fed from the power supply therethrough to the excitation windings when it is turned on and a power control circuit is arranged so as to output a control signal which acts to control on/off operation of the power feed semiconductor switch. The power control circuit is constructed so that the power feed semiconductor switch may have turn-on time set to a level sufficient to permit the rotor to get out of a negative torque region during starting of the brushless motor and turn-off time set to a level sufficient to permit the rotor to race after lapse of the turn-on time, whereby starting operation of the brushless motor by repeat of the turn-on time and turn-off time of the power feed semiconductor switch is carried out at a number of times required for permitting the rotor to be rotated in a normal or positive direction.
The power feed semiconductor switch carries out on/off operation to control power fed to the excitation windings at all times. In the present invention, the above-described control of the turn-on time and turn-off time of the power feed semiconductor switch during starting of the brushless motor permits avoidance of the above-described dead point problem. Alternatively and/or additionally, in the present invention, a control semiconductor switch arranged between the excitation windings and the power supply so as to be exclusively used for permitting feeding of power from the power supply to the excitation windings when it is turned on and a control circuit for outputting a control signal which acts to control on/off operation of the control semiconductor switch may be provided in place of or separately from the power feed semiconductor switch. Such construction likewise permits turn-on time and turn-off time of the control semiconductor switch to be controlled in a like manner. In this instance, when the power feed semiconductor switch is still arranged, it may carry out its normal operation.
In either case, setting of the turn-on time and turn-off time in such a manner as described above permits the rotor to positively get out of a negative torque region, to thereby be rotated in a normal or positive direction, in the case that the rotor is positioned in the negative torque region when the power supply is turned on. Also, when a plurality of brushless fan motors each constituting the brushless motor are arranged, there is a likelihood that one or more of the fan motors are caused to be rotated in an opposite direction by rotation of the remaining fan motors. However, repeat of the above-described operation permits the fan motors rotated in the opposite direction to escape from load due to an air pressure by the remaining fan motors, resulting in the fan motors being positively rotated in the positive direction.
The turn-on time and turn-off time of the semiconductor switch are varied depending on load conditions of the motor and/or operational conditions thereof. However, typically, the turn-on time may be set to be 0.2 second or more and the turn-off time may be set to be 0.05 second or more. Such setting of the turn-on time and turn-off time permits the dead point problem to be effectively eliminated substantially irrespective of the conditions. For a fan motor commonly used for cooling, the turn-on time may be set to be about 0.3 second and the turn-off time may be set to be about 0.1 second. In this instance, the above-described repeat of the turn-on time and turn-off time may be carried out only one time.